Following up on: T150951: Create list of privileged wiki groups and T208477: Move "privileged account' concept into MediaWiki core...

We should change WMF config from $wmgPrivilegedGroups to $wgPrivilegedGroups for variable reuse.

It doesn't add much functionality (yet), but we can reduce the "divergence" from production config, and when we plumb $wgPrivilegedGroups to do more stuff in MW Core, we get that as a beneficial side effect